Spam is the name given to unsolicited e-mail messages that are sent to people indiscriminately. These messages might also be inappropriate. For example, an e-mail message offering cheap contact lenses might be inappropriate if sent to people with unimpaired vision. A contact lens user could, however, consider the message to be of value, even though it was unsolicited. This example illustrates a major issue with such e-mail; only the recipient of a message can truly decide whether it is spam or not.
Filters are known to check e-mails for spam using several techniques including conventional filters, compliance checking and traffic anomaly detection. For example, filters include software that looks for key words (e.g. XXX, sex, etc.). When a keyword is found the e-mail message is rejected. With compliance checking, a considerable number of tools used by spammers generate “non-compliant” e-mail messages. These can be identified and ignored. Traffic anomaly detection involves monitoring where e-mail messages come from and go to, which helps to detect unusual patterns of e-mail that are often associated with mass mailings. Software can be configured to take action when specific limits are reached. All of these known filters have several problems including maintenance and false positives. Maintaining a word list for filters can become a full time job for the systems administrator. A false positive occurs when a normal, legitimate message is incorrectly identified as spam. Filtering on a keyword such as “breast” will catch all discussions on “breasts” including those concerning say, cancer treatment.